Introduction: The apparent diffusion coefficient (ADC) of water in gray and white matter decreases significantly (~35%) after global ischemia. However, the effects of ischemia on water diffusion and its anisotropy in nerve are unknown. The purpose of this study was to measure water diffusion and its anisotropy in the optic and trigeminal nerve after cardiac arrest in the rat. Methods: Cardiac arrest was induced in anesthetized Sprague-Dawley rats (n=7) by an i.v. injection of potassium chloride. Core temperature was maintained at 3711!C by warm air circulation. MRI diffusion measurements were performed on a GE CSI 2 T spectrometer prior to cardiac arrest and at 20, 40, 60, and 90 min after cardiac arrest. In order to investigate anisotropic diffusion and since the nerves are in approximate alignment with the Z direction, the ADC values parallel (//) and perpendicular (^) to the length of the nerve were obtained with the diffusion gradients applied along Z and X (or Y), respectively. Results and Conclusion: ADC(//) and ADC(^) decreased by 44 and 46%, respectively, in the optic nerve, and by 24 and 26%, respectively, in the trigeminal nerve at 90 min post cardiac arrest, relative to the ADCs prior to cardiac arrest. Interestingly, the rate of approach to the minimum ADC was slower than in the cortex. This is in agreement with the slower decrease of the ADC in white matter relative to gray matter after cardiac arrest. Despite the absolute reductions in ADC, anisotropy {ADC(//)/ADC(^)} was preserved throughout the 90 min after cardiac arrest and it remained at ~2.9 for both the optic and trigeminal nerves. Therefore, changes in diffusional anisotropy would not be expected at the acute stage of ischemia.